Humans and higher animals receive much more information from their sensory periphery than they can process in detail by their central nervous system. Selective attention is the triage process by which incoming information is divided into the behaviorally most relevant parts, which require detailed processing, and all others that are suppressed. The goal of this research is to understand the neural mechanisms underlying tactile selective attention which will be studied by single unit recordings and local field potential recordings in awake behaving monkeys. The first Aim is to determine the limits of granularity of attentional selection. What are the limits of selection in space (attend to different fingers or to different hands), submodality (attend to vibration or to form) and time (changes within the duration of one trial), and what are the neuronal correlations of this attentional selection? We will specifically measure changes in mean firing rate and in correlations between neurons (synchrony). Aim 2 is to determine the influence of attentional selection on the responses of identified neuronal subpopulations, including the functional populations defined by the cortical layers and by the postsynaptic effects of neurons (excitation or inhibition). We will also determine whether all neurons showing synchrony are part of one population or whether there are several subpopulations whose members are synchronous with each other but not with members of other subpopulations. Two theoretical models of the role of neural synchrony in perception (1: tagging of the attended stimulus, 2: tagging of different parts of a perceived stimulus) make different predictions for the number of subpopulations and this analysis will test those predictions. Aim 3 is to study the mechanisms underlying the generation of synchrony. One hypothesis that will be tested is phase locking of action potentials to a periodic function (oscillations).